On the Origin of the High Star-Formation Efficiency in Massive Galaxies at Cosmic Dawn

TL;DR

This study uses high-resolution cosmological simulations to explore the physical mechanisms behind the high star-formation efficiency in massive galaxies during Cosmic Dawn, emphasizing the roles of feedback, turbulence, and gas density.

Contribution

It introduces a physically-motivated turbulence-based star formation model in simulations of high-redshift galaxies, providing insights into the factors influencing star-formation efficiency.

Findings

High local SFE (>10%) due to dense gas and reduced SN feedback efficiency.

Global SFE is mainly regulated by feedback-driven outflows, not local conditions.

Simulated galaxy properties align with observations of massive galaxies at Cosmic Dawn.

Abstract

Motivated by the early excess of bright galaxies seen by JWST, we run zoom-in cosmological simulations of a massive galaxy at Cosmic Dawn, in a halo of $10^{11} M_\odot$ at $z = 9$, using the hydro-gravitational code RAMSES at an effective resolution $\sim 10~{\rm pc}$. We investigate physical mechanisms that enhance the star-formation efficiencies (SFEs) at the high gas densities of the star-forming regions in this galaxy ($\sim 3\times 10^3~{\rm cm^{-3}}$, $\sim 10^4~M_\odot/{\rm pc^2}$). Our fiducial star formation recipe uses a physically-motivated, turbulence-based, multi-freefall model, avoiding ad hoc extrapolation from lower redshifts. By $z = 9$, our simulated galaxy is a clumpy, thick, rotating disc with a high stellar mass $\sim 3\times 10^9~M_\odot$ and high star formation rate $\sim 50~M_\odot/{\rm yr}$. The high gas density makes supernova (SN) feedback less efficient, producing a high local SFE $\gtrsim 10\%$. The global SFE is set by feedback-driven outflows and only weakly correlated with the local SFE. Photoionization heating makes SN feedback more efficient, but the integrated SFE always remains high. Intense accretion at Cosmic Dawn seeds turbulence which reduces local SFE, but this only weakly affects the global SFE. The star formation histories of our simulated galaxies are similar to observed massive galaxies at Cosmic Dawn, despite our limited resolution. We set the stage for future simulations which treat radiation self-consistently and use a higher effective resolution $\sim 1~{\rm pc}$ that captures the physics of star-forming clouds.